468 PALEONTOLOGICAL REPORT OF GEOLOGICAL SURVEY. 



Re.tepe.ra Archimedes, spread out horizontally, and almost -constituting 

 the entire mass. Further from the base of the bed are found segre- 

 gations, broken and irregular bands and patches of earthy ferruginous 

 limestone. This alternation of limestones and shale beds continues to 

 the top of the mass. 



It is 1 from the segregated masses, at the top of the first sixty feet of 

 this intercalated calcareous bed, that some of the fossil forms selected 

 for description were obtained ; and, so far as it is at present known, cer- 

 tain remarkable forms of this bed have never been found extending ei- 

 ther above or below its geological horizon. 



The vertical range of the first organic form which will be described 

 is not more than five or six feet. Two crashed specimens were found 

 in 1845; others, again, in 1852. Having recently obtained some 

 quite perfect specimens, it is proposed to describe them under the 

 name of Pentremites obesus. 



CMNOIDEA. 



GENUS PENTREMITES. Say. 



In the year 1820 the genus Pentremites was proposed by Mr. 

 Thomas Say* in which were placed certain fossil forms, then, for the 

 first time, described. Since the erection of the genus it has been gen- 

 erally recognized, and many species have been added by different au- 

 thors. One of the latest authorities, Messrs. De Koninck and Le Hon, 

 state the genus under the following formula, viz: 

 Basal pieces, 3, one less than the two others. 



Radial pieces, lX^, forked, large. 

 Interradial, 1X5, small lanceolate. 



Pseudambulacrse, 1x5, 

 Mouth, 1, central. 



Anal, 1, lateral. 



Ovarial openings, 2X-5, situated around the mouth. 



By a careful examination of well preserved specimens, (not silicifi- 

 ed,) of the different species of this genus, including the typical spe- 

 cies, upon which the genus was founded, it may be seen that the formu- 

 la above quoted should be amended. Pentremiies floreaiisjglobosiis, py- 



•See vol. ii, Si!!iman'8 Journal) p. 36, and American Journal of Science and Arte, vol. 3. 



